The present disclosure is related to the field of lithographic printing, and more particularly, to the manufacturing of a lithographic printing member that is used in waterless offset printing systems.
In offset lithography printing, an image is presented on a lithographic printing member, such as a printing plate or a printing cylinder, wherein the imaged area has a pattern of ink-accepting (oleophilic and/or hydrophobic) and ink-repellent (oleophobic and/or hydrophilic) surface areas. There are two general offset printing methods, a wet method and a dry method (waterless). The wet method, which is the traditional method, uses a fluid that is dampened (or “fountain”) to the printing member prior to the ink. The fluid, such as water, covers the ink-repellent surface areas and repels the ink that is applied later to the printing plate, but does not affect the oleophilic character of the image areas. Therefore, traditionally, the non-image areas are called hydrophilic areas while the ink-accepting areas are called hydrophobic areas.
The typical dry or waterless lithographic printing member has at least two layers with at least two layers having a different affinity for printing ink. For instance, one layer is made of or includes an oleophobic material that rejects ink, such as silicone rubber. Another layer is made of or includes an oleophilic material such as polyester. Therefore, in dry printing systems, the plate is simply inked and the ink is carried by the oleophilic areas that were exposed imagewise.
It should be noted that the terms “printing member”, “printing plate”, “lithographic printing member” and “plate” are used interchangeably herein. It also should be noted that the terms “ink-accepting” and “oleophilic” are used interchangeably herein and it should be noted that the terms “ink-repellent” and “oleophobic” are used interchangeably herein.
In waterless printing methods, the image is patterned over the plate creating ink-accepting (oleophilic) and ink-rejecting (oleophobic) surface areas. Ink that is applied to the lithographic printing member is carried by the oleophilic areas and is transferred to a recording medium in the image-wise pattern. Typically, the inked printing member first makes contact with an intermediate surface called a blanket cylinder, which, in turn, applies the image ink to the paper or other recording medium.
There are several ways to expose the image over a printing member. Some of those methods involve direct computer to plate (CTP) equipment. Common imaging methods of a printing member exposes the printing member image-wise by a computer control laser radiation, usually using infrared (IR) or near IR radiation. The image-wise IR radiation elevates the temperature of the IR absorber and deanchoring the top oleophobic layer.
For example, Great Britain patent 1489308 (Eames) describes a dry planographic printing plate comprising an ink receptive substrate, an overlying silicone rubber layer, and an interposed layer comprised of laser energy absorbing particles (such as carbon particles) in a self-oxidizing binder (such as nitrocellulose). The described planographic plates are exposed to focused near IR radiation with a YAG laser. The absorbing layer converts the infrared energy to heat thus partially loosening, vaporizing or ablating the absorber layer and the overlying portions of the silicone rubber layer. Similar plates are described in Research Disclosure 19201, 1980 as having vacuum-evaporated metal layers to absorb laser radiation in order to facilitate the removal of a silicone rubber overcoated layer. These plates are described as being developed by wetting with hexane and rubbing. Other publications describing ablatable printing plates include U.S. Pat. No. 5,339,737 (Lewis et al.), U.S. Pat. No. 5,353,705 (Lewis et al.), U.S. Pat. No. 5,378,580 (Leenders).
Many of the currently available ablatable printing plates designed to absorb laser energy include an IR absorbing substance, such as a pigment and/or dye, and self-oxidizing polymer binders such as nitrocellulose and a crosslinking agent like melamine.
All ablative plates, after imaging, undergoing a cleaning process to remove residue of the ablated silicone from the plate. The cleaning process can include the application of solvents and can be a wet or dry process. Solvent cleaning processes are not user friendly or ecologically friendly. Water based or dry cleaning of the plate is a more suitable ecological means of cleaning the plate, but it requires effort and time to release all residue of silicone from the image, especially in the case of high resolution imaging such as 300 lpi or more.
A few examples of existing printing members and methods for manufacturing them are now presented as further background of the related technology. One example of prior art IR ablative waterless printing plates utilize a silicone top layer, a second layer or imaging layer including IR sensitive laser absorbing material and a substrate. The top layer is silicone, like polydimethylsiloxane rubber, with a thickness of about 2 microns. The second layer is made of a polymer and/or a cross-linkable resin, IR absorbing pigment or dye, and a cross-linking agent. In many existing plates, the polymer or resin layer can be made up of nitrocellulose, which operates as the ablating agent layer as it is self-oxidized by thermal irradiation. Other polymers are also described for such applications, such as derivatives of vinyl terpolymer, polyvinylidenchloride, cyanoacrylate polymer binder etc. Usually, the thickness of such layers is in the range of 0.5-1 microns. In other prior art printing member, the second layer can be constructed of metal, metal oxide or a combination thereof, usually applied in vacuum.
Typically, the substrates described in the prior art is made from aluminum or polyester film and is either clear or white. In case of aluminum substrates, an insulating layer is applied between the substrate and the imaging IR sensitive layer. This insulating layer serves to prevent the imaging layer from dissipating the thermal energy provided by the laser to the metal or substrate. This insulating layer typically has oleophilic ink receptive properties.